staticcheck (#313)

* CI: use staticcheck for linting

This commit switches the linter for Go code from golint to staticcheck.
Golint has been deprecated since last year and staticcheck is a
recommended replacement.

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

* revendor

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

* cmd,pkg: fix lint warnings

Signed-off-by: Lucas Servén Marín <lserven@gmail.com>

vendor/honnef.co/go/tools/unused/edge.go

@@ -0,0 +1,59 @@
+package unused
+
+//go:generate go run golang.org/x/tools/cmd/stringer@master -type edgeKind
+type edgeKind uint64
+
+func (e edgeKind) is(o edgeKind) bool {
+	return e&o != 0
+}
+
+const (
+	edgeAlias edgeKind = 1 << iota
+	edgeBlankField
+	edgeAnonymousStruct
+	edgeCgoExported
+	edgeConstGroup
+	edgeElementType
+	edgeEmbeddedInterface
+	edgeExportedConstant
+	edgeExportedField
+	edgeExportedFunction
+	edgeExportedMethod
+	edgeExportedType
+	edgeExportedVariable
+	edgeExtendsExportedFields
+	edgeExtendsExportedMethodSet
+	edgeFieldAccess
+	edgeFunctionArgument
+	edgeFunctionResult
+	edgeFunctionSignature
+	edgeImplements
+	edgeInstructionOperand
+	edgeInterfaceCall
+	edgeInterfaceMethod
+	edgeKeyType
+	edgeLinkname
+	edgeMainFunction
+	edgeNamedType
+	edgeNetRPCRegister
+	edgeNoCopySentinel
+	edgeProvidesMethod
+	edgeReceiver
+	edgeRuntimeFunction
+	edgeSignature
+	edgeStructConversion
+	edgeTestSink
+	edgeTupleElement
+	edgeType
+	edgeTypeName
+	edgeUnderlyingType
+	edgePointerType
+	edgeUnsafeConversion
+	edgeUsedConstant
+	edgeVarDecl
+	edgeIgnored
+	edgeSamePointer
+	edgeTypeParam
+	edgeTypeArg
+	edgeUnionTerm
+)